Cytochrome P-450a is a P-450 isozyme showing a high degree of regiospecificity of the 7 alpha-hydroxylation of steroids. The steric and electronic binding properties responsible for this specificity will be probed by evaluating the metabolism of several steroid analogs or fragments. Models used in this study include aliphatic and unsaturated ring systems steroid fragments, and a potential heme-binding optically active steroid analog. The regioselectivity, chiral and prochiral selectivity, and kinetics of the metabolism of these substrates will be used to evaluate the geometric properties of the P-450a active site.